1. Technical Field of the Invention
This invention relates to the construction of wells. The invention further relates to primary cementing operations involved in the construction of wells penetrating subterranean formations. This invention still further relates to hydraulic cementing compositions and methods of using them to perform primary cementing operations.
2. Problems Solved
During the construction of a well, such as an oil and gas well, it is known in the art to place a volume of a water slurry of a hydraulic cement into the annular space defined between the walls of the borehole penetrating a subterranean formation and the exterior surface of the well casing suspended therein. It is also known in the art to place the slurry into the annular space defined between the walls of concentric pipes--such as a well casing and a liner--suspended in the borehole. Subsequent to placement of the slurry further construction operations in the borehole, such as drilling, are suspended for a time sufficient to permit the slurry to set, that is to solidify, to thereby form a mass of hardened cement in the annulus. The mass of hardened cement is referred to as the sheath. The cementing procedure thus described leading to and terminating with the construction of the sheath is referred to as primary cementing. The objectives of the sheath are to provide physical support and positioning of the casing in the borehole; to bond the casing to the walls of the borehole; to prevent the movement of fluid (liquid or gas) between formations penetrated by the borehole; and to prevent fluid from completely escaping the well at the surface of the earth.
In addition to the above objectives, it is desirable that the slurry set and develop sufficient compressive strength in as short a period of time as possible after placement thereof in order to avoid excessive delay in performing further well construction operations. It is a generally accepted standard in the well drilling industry that the compressive strength of set hydraulic cement in the sheath preferably shall be at least about 500 pounds per square inch before further operations may be conducted, wherein the time required to develop the strength, measured from the completion of the placement of the slurry in the annulus, preferably shall not exceed about 24 hours. This time delay is referred to in the art as waiting-on-cement or simply as WOC.
When a slurry of hydraulic cement is positioned in the desired location in the annulus, the slurry movement rapidly changes from a moving, or dynamic, condition when being placed, to a resting, or static, condition after being placed. The slurry, when in the static condition and not yet set, must immediately resist movement of formation fluid through the slurry. Such movement, which is generally gas moving under the influence of formation pressure, is referred to as annular gas migration or simply as gas migration. In addition, the slurry itself, when in the static condition, must resist any movement by it into subterranean zones with which the slurry is in communication. Such movement is generally due to the influence of hydrostatic pressure of the slurry on the zone and is referred to as fall back because the height of slurry in the annulus is reduced by the quantity of slurry penetrating the zone.
Gas migration in the slurry leads directly to gas migration in the sheath, and can cause poor bonding of cement, casing and borehole. Fall back can result in a reduction of the quantity of sheath required to position and support the casing in the borehole. Accordingly, gas migration through the slurry and fall back of the slurry can frustrate the stated objectives of the sheath.